[3.04] The most obscured supermassive black holes and the Cosmic X-ray background

A. Comastri (INAF-Osservatorio Astronomico Bologna)

Deep hard X-ray surveys have resolved a significant fraction
of the hard X-ray background into individual sources.
Despite intensive multiwavelength follow-up observations a
sizable fraction of hard X-ray sources (presumably the most
obscured and at high redshift) remain unidentified. I will
discuss several approches developed by our team (the
HELLAS2XMM collaboration) to investigate the nature of these
sources and the consequences for the study of their space
density and evolution as well as for the modelling of the
hard X-ray background. I will also discuss the need for
X--ray observations above 10 keV to search for the sources
of the hard X-ray background close to the peak of its
emission (30 keV).